The present invention relates to a biaxially oriented laminated polypropylene (BOPP) film having a printable urethane coating.
BOPP films are widely used films because they have good stiffness, strength, optical properties (low haze and high gloss), and moisture barrier properties. Users of such films are continually seeking structures with improved printability. Because of their olefinic nature, typical BOPP constructions have low surface energy and require treatment (corona, flame, etc.) in order to be printable.
Urethane dispersions and blends of urethane dispersions and acrylic dispersions are known to function as useful primers for coatings and inks. Commercial products are available. In general, primers are materials that have low glass transition temperatures (Tg), the use of which often results in heavily blocked rolls. Films produced in a standard coating operation using the waterborne urethane dispersions without a topcoat result in a very tacky film that blocks on the machine winder and renders the roll unusable. The present invention is directed to a film where a low Tg coating is provided without the blocking tendency.
Waterborne acrylic dispersions blended with waterborne urethane dispersions are excellent coatings for printing using a variety of inks and also have good water resistance properties. These coating formulations typically require the addition of crosslinking agents (crosslinkers), for example, at 3-5% by weight, in order to harden the coating to pass ice chest and pasteurization tests. Crosslinking agents often hydrolyze in the mixed batch. This hydrolyzation requires additional work for a coating operation in that small batches must be mixed or the batch must be re-inoculated with the crosslinking agent when ready to be used. Additionally, if the formulation is created incorrectly, one may get unwanted crosslinking reactions causing the mixed material to be useless. Elimination or reduction of the crosslinking agent is desired.
The present invention is directed to a film and a method for making the film that resolves the above-stated issues.
Related prior art patents include the following:
U.S. Pat. No. 4,525,419 (1985) describes a primer coated, oriented plastic (PET) film where the primer is a waterborne copolyester dispersion. The resultant primed plastic film is disclosed as having improved adhesion to aqueous and solvent based coatings that are applied to the film. The coating may be applied before the film is stretched or after the film is uniaxially stretched, but before the film is stretched in the transverse direction.
U.S. Pat. No. 4,225,644 (1980) teaches a method of applying in-line coatings to oriented polypropylene film. The properties generated are lubricity, anti-blocking, and anti-scratch.
U.S. Pat. Nos. 5,156,904 (1992) and 5,453,326 (1995) demonstrate application of polyethyleneimine and polyamido-polyethyleneimine to improve extrusion lamination and ink adhesion. Both patents use in-line coating technology on polyester film.
All references cited herein are incorporated herein by reference in their entireties.